1. Field of Invention
The present invention relates to a multi-phase switching regulator, a driver circuit of a multi-phase switching regulator and a control method of a multi-phase switching regulator, in particular to such multi-phase switching regulator, driver circuit and control method wherein the gain of a current balance circuit can be adjusted to avoid voltage overshoot and undershoot when the phase number of power stages is increased or decreased.
2. Description of Related Art
FIG. 1 shows a schematic diagram of a prior art multi-phase switching regulator. As shown in FIG. 1, a first phase PWM (pulse width modulation) controller 11 and a second phase PWM controller 21 respectively output a signal PWM1 and a signal PWM2 to control corresponding power transistors of a first phase power stage 12 and a second phase power stage 22 so that an input voltage Vin is converted to an output voltage Vout. A feedback circuit 13 generates a feedback signal related to the output voltage Vout and supplies the feedback signal to the first phase PWM controller 11 and the second phase PWM controller 21. A current balance circuit 14 receives a first signal related to a first inductor current I1 and a second signal related to a second inductor current I2, and outputs a current balance signal to the first phase PWM controller 11 and the second phase PWM controller 21. Accordingly, the first phase PWM controller 11 and the second phase PWM controller 21 respectively control the first phase power stage 12 and the second phase power stage 22 to balance the current I1 and the current I2, while the output current Iout is maintained at a desired target. The power stages 12 and 22 may be, for example but not limited to, synchronous or asynchronous power conversion circuits such as buck converters, boost converters, inverting converters, or buck-boost converters, as shown in FIGS. 2A-2J.
In light-load condition wherein relatively lower output current is required, not all of the power stages need be active to supply currents to the load. At least one power stage can be switched off (phase off) so as to reduce the operation loss of the circuit. When such a condition occurs, the prior art method instantly and completely switches off the second phase power stage 22. That is, a phase off signal is inputted to the second phase PWM controller 21 so that the duty ratio of the signal PWM2 immediately drops to zero. However, it requires a period of time for the current I1 of the first phase power stage 12 to reach to a level required by the load. This causes an undershoot in the output voltage and easily result in system crash or other problems.
U.S. patent publication No. 2008/0272752 discloses a multi-phase switching regulator which can switch off an operating power stage while maintaining the output voltage. However, this prior art can be applied only to a multi-phase switching regulator with at least three phases. In addition, when a power stage is to be switched off, its corresponding PWM signal is added to the PWM signal of the first phase power stage. Thus, output voltage undershoot will still occur because there is a limit for the power stages remain operating to ramp up their currents.
On the other hand, when the load becomes heavy and requires relatively higher output current, the circuit needs to restore one or more power stages which are previously switched off, so as to supply more current to the load. That is, a “phase on” operation is needed (i.e., at least one inactive power stage needs to be switched on). When such a condition occurs, the prior art switches on the second phase power stage 22 (assuming that the second phase power stage 22 has been switched off previously); a phase on signal is inputted to the second phase PWM controller 21 to activate the second phase power stage 22, and the current balance circuit 14 starts to detect the inductor currents of the operating phases, and outputs a current balance signal to the controllers of the operating phases for current balance. However, in the transition period when a previously inactive power stage is just switched on, the inductor currents of the operating power stages are certainly higher than the target values for current balance, so the current balance circuit 14 will reduce the currents of these power stages. Consequently, the output current, which is the sum of all inductor currents of all phases, is not sufficient to support what is required by the load, and hence an output voltage undershoot often occurs during such transition period to cause system crash or other problems.
U.S. Pat. No. 7,023,182 discloses a multi-phase control system which forces the duty ratio of the PWM signal outputted from a PWM controller to be increased or decreased according to a signal relating to output current. The duty ratio of a power stage which is newly activated is increased by a fixed amount, while the duty ratio of an operating power stage is decreased by a fixed amount, to improve the output voltage undershoot resulting from activating a new phase. It is not flexible to force the duty ratios to be increased or decreased in this manner, and the output voltage undershoot still cannot be completely avoided.
In view of above, the present invention overcomes the foregoing drawbacks by providing a multi-phase switching regulator and a driver circuit and a control method therefor, wherein the gain of a current balance circuit is adjusted to avoid voltage overshoot and undershoot when the phase number of power stages is increased or decreased.